Inkjet recording head

ABSTRACT

An inkjet recording head for ejecting ink in ink channels by deformation of the piezoelectric element, comprising: a partition wall, formed by making grooves in an actuator substrate made of a polarized piezoelectric element, for forming longitudinal side surface of a plurality of ink channels; a cover plate adhered on an upper surface of the partition wall to form a top surface of the plurality of ink channels; a nozzle plate, adhered on a front side surface of the actuator substrate, having a nozzle hole for ejecting ink, wherein the cover plate is made of a machinable ceramics, which has a higher thermal conductivity than that of the piezoelectric element, and where Lc and Lp respectively represent the linear thermal expansion coefficient of the cover plate and the piezoelectric element, the relationship of, |Lc−Lp|≦5×10 −6 /° C. is satisfied, and a top plate having higher thermal conductivity than the cover plate is provided on the cover plate.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 10/657,402, filed on Sep. 8, 2003 now abandoned,the entire contents of which are incorporated herein by reference. TheSer. No. 10/657,402 application claimed the benefit of the date of theearlier filed Japanese Patent Application No. JP 2002-267308 filed Sep.12, 2002 priority to which is also claimed herein.

BACKGROUND OF THE INVENTION

The present invention relates to an inkjet recording head with a simplestructure that can solve the problems caused by a decrease of inkviscosity by dissipating the drive heat generated during ink ejection,which occurs during high speed drive of the recording head. (PRIOR ART)

Conventionally, a so-called shear mode type inkjet recording head(hereinafter, referred as a shear-mode head) is known in the industry,in which an actuator substrate is structured with a plurality of grooveson a polarized piezoelectric element, a plurality of pressure generationrooms partitioned by said piezoelectric element are formed by adhering acover plate onto the upper surface of said actuator substrate, saidpiezoelectric element is deformed by applying an electric voltagebetween the adjacent pressure generation rooms, and ink is ejected fromnozzle holes provided in a nozzle plate.

In this shear-mode head, since ink channels to be filled with ink areformed in the piezoelectric element, when the piezoelectric elementgenerates heat, this heat is transferred to and heats the ink. If thetemperature of the ink is raised by heating, the viscosity of the ink isreduced and ink ejection speed increases and the landing position of theink tends to deviate from the targeted position to cause significantdegradation of image quality.

For this reason, in the shear-mode head, without employing a positiveheat dissipating measure, or with an insufficient heat dissipatingmeasure, the heat generated in the piezoelectric element has no root ofdissipation, and since the heat is transferred to the ink, the viscosityof the ink decreases, the ejection speed of ink drops increases and thiscauses landing position errors with regard to the recording mediummoving at constant rate, and resulting in degradation of image quality.

This heat generation phenomenon affects sensitively the image quality.For example, when ink drops of 20 pl are continually ejected for 2 secby the drive voltage of 20 V, at a frequency of 17 kHz—this conditioncan be expressed more specifically based on an actual case—when an inkhead carriage moves and prints reciprocally between the edges of a 1350mm wide width recording medium, with the rate of 600 mm/sec, for theprinting of one onward way or homeward way it takes about 2 sec., inthis condition, the ink ejection speed at the end of 2 sec. of ejectionincreases more than 0.1 m/sec compared to the start of the ejection, andthe printed image density varies more than 0.01. This is due to heatgeneration of the head, heat transfer to the ink and decrease of the inkviscosity.

During the time when ink ejection stops for switching the ink carriagemoving direction from onward to home ward, the temperature of the headdecreases, and after switching the direction, printing is againconducted resulting in variation of image density of more than 0.01between the start and end of printing. A density difference of 0.01seems to be a rather small value, however, this density differenceappears at adjacent positions in the right and left end of thewide-width recording medium, and this density variation can be visibleto the naked eye. In order to make this density variation invisible tothe naked eye, it is necessary to reduce the ink ejection speed raisedue to heat generation of the head to less than 0.1 m/sec., and toreduce the density variation to less than 0.01, during 2 sec. ofcontinuous ink ejection.

The raising rate of the ink ejection speed increase with respect to theink temperature increase varies between low viscosity ink and highviscosity ink. For example, as for the high viscosity ink having theviscosity of 10 cp at room temperature, it is known that the inkejecting speed increases 0.3 m/sec. for every 1° C. temperature raise.Therefore, in cases where a wide-width recording medium is printed fromedge to edge of the width, the ink temperature increase in the head isnecessary to be restricted within 0.3° C. If ink ejection is continuedfor a long time, the heat accumulated in the piezoelectric element istransferred to the ink, and the ink temperature gradually increases.Usually, a head is structured such that a thermistor is provided on thehead which detects the ink temperature to control the drive voltage ofthe head to keep the ink ejecting speed constant, however, there areabout 10 seconds delay for its response, and this can not adequatelyrespond to the temperature increase which occurs during one line ofprinting with not more than 10 sec.

Although, temperature increase of the head by the continuous inkejection during one line printing with about 2 seconds for wide-widthrecording medium is expected to be rather small, the preferablecountermeasure for preventing the temperature increase during such ashort time is to improve dissipation of the head to prevent the heatgenerated in the piezoelectric element from flowing toward the ink.

In the prior art, the technologies are known in which a drive circuit ICis incorporated inside a high heat-conductive and electricallyinsulative ceramic board to dissipate the heat generated by the IC(refer to patent article 1), and other technology in which a heatgenerating element is adhered on to a high heat-conductive board by ahigh heat-conductive film adhesion tape (refer to patent article 2) areknown.

However, only a countermeasure such that the member contacting thepiezoelectric element is constituted with high heat-conductive materialfor dissipating the heat of the piezoelectric element is not sufficientto overcome the following problems.

Namely, since in the shear-mode head, ink ejection amount and inkejection frequency are largely determined by the length of the inkchannel, in order to eject sufficiently small ink drops at highfrequency, it is necessary to make the length of ink channel not morethan 5 mm. For this reason, a piezoelectric member with a length ofseveral cm is necessary to be cut out to make prescribed length of themembers, after the member is ground to form grooves, electrodes areformed, and a cover plate is adhered onto the top surface of the formedink channels. In cases where physical properties of the piezoelectricelement and that of the cover plate are greatly different with eachother, for example, in the case of ceramics material harder than thepiezoelectric element being used for the cover plate, if grindingconditions are set based on the harder ceramics material, thepiezoelectric element, which is a less hard material, can be excessivelyground resulting in excessively large grooves for ink channels. On thecontrary, if the grinding conditions are set based on the piezoelectricelement of less hard material, the harder cover plate material cannot becut well enough. Since a nozzle plate is adhered on this cut surface,forming a nozzle for ejecting ink, if the cut surface is rugged thenozzle plate cannot keep a flat surface and this results in the problemof deflecting the ink ejection direction from the nozzle.

For this reason it is required to adhere the nozzle plate only after thecut surface of the piezoelectric element is polished and smoothed. Thepolishing requires a considerably long time and is a troublesomeprocess, and further, can lead to problems of clogging and contaminationin the ink flow path during the process.

For the piezoelectric element, PZT is frequently used. Since PZT has aYoung's modulus of about 50 Gpa, which is a rather small value forceramics, if it is ground with a diamond cutter, a smooth ground cutsurface can be obtained. Other popular ceramics, alumina for example,has a high Young's modulus of about 300-400 Gpa, therefore, the memberobtained by adhering the PZT and the alumina is difficult to cut bygrinding, and a smooth cut surface cannot be obtained, which requires anadditional time consuming process to polish the cut surface.

As for the cover plate, using the same material as the piezoelectricelement is preferable from the viewpoints that thermal expansion doesnot need to be considered and a smooth cut surface can be obtained.However, the piezoelectric element, for example, consisting of PZT has alow thermal conductivity of 1.5-2.0 W/mK, and the heat generated insidethe piezoelectric element is hard to dissipate. Namely, if the samematerial as the piezoelectric element is used for the cover plate, theink channels are enclosed with materials of low thermal conductivity, sothe heat generated in the piezoelectric element is hard to dissipate,which eventually leads to the increase of ink temperature.

Further it is known to use a ceramics with high thermal conductivity asthe cover plate covering the upper surface of the ink channels made onthe piezoelectric element, and to adhere them with an adhesive with highthermal conductivity (refer to patent article 3), however, said problemsregarding the grinding process are not mentioned in the prior art.

-   -   Patent article 1: TOKKAI-HEI NO. 10-217454    -   patent article 2: TOKKAI NO. 2001-150680    -   Patent article 3: TOKKAI NO. 2000-135788

The objective of the present invention is to improve heat dissipationfrom the piezoelectric element by using a material with higher thermalconductivity than the piezoelectric element as the cover plate, and toprevent deformation or separation of the inkjet head which used to occurduring the use of the head or in the manufacturing process, by usingmaterials having a similar thermal expansion coefficient with thepiezoelectric element, and further to provide an inkjet head of highreliability, which does not require in its manufacturing process topolish the ground cut surface of the member constituted by adhesion ofthe piezoelectric element and the cover plate.

SUMMARY OF THE INVENTION

The above-described objectives are attained by the following features.

An inkjet recording head for ejecting ink in ink channels by deformationof the piezoelectric element, comprising: a partition wall, at least apart of which is formed with a piezoelectric element, for partitioning aplurality of tubular ink channels; a top wall for forming a top surfaceof the plurality of tubular ink channels by shielding an upper part ofthe plurality of tubular ink channels; and a bottom wall for forming abottom surface of the plurality of tubular ink channels by shielding thebottom part of the plurality of tubular ink channels; wherein, at leasta part of the top wall and the bottom wall is made of AlN-BN.

An inkjet recording head for ejecting ink in ink channels by deformationof the piezoelectric element, comprising: a partition wall forpartitioning the plurality of tubular ink channels; a top wall forforming a top surface of a plurality of tubular ink channels byshielding an upper part of the plurality of tubular ink channels; and abottom wall for forming a bottom surface of the plurality of tubular inkchannels by shielding the bottom part of the plurality of tubular inkchannels; wherein, at least a part of the top wall and the bottom wallis formed of a piezoelectric element, and at least a part of the topwall and/or the bottom wall is made of AlN-BN.

The inkjet recording head according to (1) or (2), wherein the part ofthe top wall and the bottom wall made of AlN-BN is thermally connectedto a heat sink.

The inkjet recording head according to (3), wherein the part of the topwall and/or the bottom wall made of AlN-BN is adhered to the heat sinkvia an epoxy type adhesive agent including Ag particles.

The inkjet recording head according to (4), wherein a layer thickness ofthe epoxy type adhesive agent is 50 to 70 μm.

The inkjet recording head according to (3), wherein a thickness of theheat sink is 1.0 to 10.0 mm.

The inkjet recording head of according to (1), wherein the part of thetop wall and the bottom wall made of AlN-BN is adhered to the partitionwall via an epoxy type adhesive agent including particles of one ofaluminum-nitride, alumina and silica.

The inkjet recording head of according to (2), wherein the partitionwall is formed of AlN-BN, and the partition wall is adhered to the partof the top wall and the bottom wall formed of a piezoelectric element,via an epoxy type adhesive agent including particles of one ofaluminum-nitride, alumina and silica.

The inkjet recording head according to (7) or (8), wherein a layerthickness of the epoxy type adhesive agent including particles of one ofaluminum-nitride, alumina and silica is 5 to 10 μm.

The inkjet recording head according to (3), wherein the heat sink isprovided on a carriage, on which the inkjet recording head is installed.

The inkjet recording head according to (3) or (4), wherein the heat sinkis thermally connected to a carriage, on which the inkjet recording headis installed.

Further, the above-described objectives are attained by the followingfeatures.

(101) An inkjet recording head, in which an actuator substrate isstructured by forming a plurality of grooves on a polarizedpiezoelectric element, a plurality of pressure generation rooms (inkchannels) partitioned with said piezoelectric element (partition wall)are formed by adhering a cover plate onto the upper surface of saidactuator substrate, and said piezoelectric element is deformed byapplying an electric voltage onto the piezoelectric elements providedbetween adjacent pressure generation rooms, for ejecting ink from nozzleholes made in a nozzle plate; the inkjet head is characterized in thatthe cover plate is made of a machinable ceramics, which has a higherthermal conductivity than that of the piezoelectric element, and when Lcand Lp respectively represent the linear thermal expansion coefficientof the cover plate and the piezoelectric element, the relationship of,|Lc−Lp|≦5×10⁻⁶/° C. is satisfied, anda top plate having higher thermal conductivity than the cover plate isprovided on the cover plate.

(102) The inkjet recording head of feature (101), characterized in thatthe Young's modulus of the cover plate is 50-200 GPa.

(103) The inkjet recording head of features (101) or (102),characterized in that the flexural strength of the cover plate is notless than 100 Mpa.

(104) The inkjet recording head of features (101), (102), or (103)characterized in that the Vickers hardness is not greater than 5.0 GPa.

(105) The inkjet recording head of any one of features (101) to (104),characterized in that the dielectric constant (ε) of the cover plate isnot greater than 100.

(106) The inkjet recording head of any one of features (101) to (105),characterized in that the top plate is a support member for mounting therecording head onto a carriage.

(107) The inkjet recording head of feature (106), characterized in thatthe thickness of the top plate is 1.0-10.0 mm.

(108) The inkjet recording head of any one of features (101) to (107),characterized in that the adhesive agent used for adhering the coverplate and the top plate comprises epoxy adhesive added with Agparticles.

(109) The inkjet recording head of any one of features (101) to (108),characterized in that the adhesive agent used for adhering the coverplate and the piezoelectric element comprises epoxy adhesive added withaluminum nitride, alumina, or silica.

(110) The inkjet recording head of any one of features (101) to (109),characterized in that the thickness of the adhesive layer between thetop plate and the cover plate is 50-70 μm, and the thickness of theadhesive layer between the piezoelectric element and the cover plate is5-10 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings which are meant to be exemplary,not limiting, and wherein like elements are numbered alike in severalFigures, in which:

FIG. 1 is an elevational sectional view showing one example of theinkjet recording head of the present invention.

FIG. 2 is a sectional view taken along the line II-II of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, embodiments of the present invention will be explainedreferring to the drawings.

FIG. 1 is an elevational section view showing one example of the inkjetrecording head of the present invention, and FIG. 2 is a sectional viewtaken along the line II-II of FIG. 1.

In FIGS. 1 and 2, H denotes a recording head, 1 denotes an actuatorsubstrate, 2 denotes a nozzle plate, 3 denotes nozzle holes, 4 denotesink channels as pressure generation rooms, 5 denotes side walls, 6denotes a cover plate, 7 denotes a manifold, and 8 denotes a FPC(Flexible Print Circuit) board.

Incidentally, the recording head shown in the embodiment is structuredsuch that two of actuator substrate 1 are adhered back to back to eachother by displacing a half distance of the nozzle pitch, to formsemi-symmetrical configuration about the center line shown by a dashedline in FIGS. 1 and 2. By this structure, without changing the headwidth, number of the nozzles are increased to twice and nozzle densityis also increased to twice.

Actuator 1 is structured by adhering through adhesive agent twopiezoelectric elements 1 a and 1 b polarized in different direction, andis subjected to a grinding work with diamond blades, etc. from the upperside of piezoelectric element 1 a to form plural grooved ink channels 4of the same shape and parallel to each other. By this structure,adjoining ink channels 4 are partitioned with side walls 5 polarized inthe direction of the arrows in FIG. 1. Further, ink channel 4 has deepgroove portion 4 a, which is positioned near the exit (left side inFIG. 1) of the ink channel of actuator substrate 1, and shallow grooveportion 4 b, whose depth gradually decreases from deep groove portion 4a toward the entrance side of the ink channel (right side in FIG. 1).

Materials used for piezoelectric elements 1 a and 1 b are not speciallyrestricted only if the material deforms by application of a voltage, andany commonly known materials such as a board consisting of an organicmaterial can be used, however, a board consisting of piezoelectricnonmetallic material is preferably used. As for the piezoelectricnonmetallic board, there are for example ceramics board formed throughthe processes of molding and sintering, or a board formed without theneed of molding or sintering. As for the organic material, an organicpolymer and a hybrid material of organic polymer and inorganic materialare listed.

Regarding the ceramic board, there are PZT (PbZrO₃—PbTiO₃) and PZT addedwith a third component. As for the third component, there are Pb(Mg_(1/3)Nb_(2/3))O₃, Pb (Mn_(1/3)Sb_(2/3))O₃, and Pb(Co_(1/3)Nb_(2/3))O₃, etc. and further the ceramic board can be formedby using BaTiO₃, ZnO, LiNbO₃, and LiTaO₃, etc.

Regarding the board formed without the need of molding or sintering, itcan be formed for example by sol-gel processing or laminated substratecoating. According to the sol-gel processing, “sol” is adjusted throughchemical reactions such as hydrolysis generated by adding water, acid oralkalis into a homogeneous solution having a prescribed chemicalcomposition. Further, by subjecting the “sol” to a process such assolvent vaporization or cooling, “sol” dispersed with fine particles ofthe aimed composition or a precurser of nonmetallic inorganic fineparticles is formed and a board can be made from it. Including a processof adding a minute amount of different elements, a chemical compoundhaving uniform chemical components can be obtained by this sol-gelprocessing. As a starting material, in general, a water soluble metallicsalt such as sodium silicate or metallic alkoxide is used. The metallicalkoxide is a compound expressed by general formula M(OR)n, and having astrong basic property due to the OR group, it is easily hydrolyzed to ametal oxide or its hydrate through a condensation process, similar to anorganic polymer.

Regarding the laminated substrate coating, there is a method fordepositing from a gaseous phase, and methods for forming a ceramic boardare classified into two types, a physical evaporation method type and achemical reaction method type in a gaseous phase or on the surface ofthe substrate. Further, the physical evaporation method (PVD) type issegmented into vacuum evaporation, spattering, and ion-plating methods,etc., and the chemical reaction method type is segmented into a gaseousphase chemical reaction method (CVD) and a plasma CVD method etc. Thevacuum evaporation method as the physical evaporation method (PVD) is amethod of depositing vapor onto a substrate by heating and evaporating asubject material in a vacuum chamber. A spattering method is the methodof utilizing the phenomenon that when high energy particles collide witha target material to transfer motion energy from the collision particlesto atoms/molecules on the target surface to make them sputtered. Theion-plating method is a method of evaporating in an ionized gasatmosphere. In a CVD method, atoms, molecules or compounds includingions, which are to compose the deposition layer, are made to be ingaseous phase, and lead to a reaction zone by an appropriate carrier gasfor reaction or deposition on a heated substrate to form a depositionlayer. In a plasma-CVD method, a gaseous phase is generated by plasmaenergy, and a deposition layer is formed by a gaseous phase chemicalreaction at a relatively low temperature of 400-500° C.

On the upper surface of actuator substrate 1, cover plate 6 is adheredvia an adhesive agent to cover deep groove portions 4 a of all inkchannels 4, and on each shallow groove portions 4 b, ink inlet 4 c intoink channel 4 is formed, and manifold 7 is adhered to cover this inkinlet 4 c via an adhesive agent. Further, at the front edge surface ofactuator substrate 1, on which cover plate 6 is adhered, nozzle plate 2having nozzle hole 3 is adhered via an adhesive agent.

In each ink channel 4, metal electrodes 9 are formed through on bothside walls to on the bottom wall, and these metal electrodes extendthrough shallow groove portion to the upper surface 1 c in the rearportion of actuator substrate 1. On each of metal electrodes 9, FPC 8 isadhered via ACF (Anisotropic Conductive Film) 10 on the upper surface 1c at the rear portion of actuator substrate 1. By applying a drivevoltage onto each metal electrode 9 from a drive circuit (notillustrated) through electrodes 8 a formed on the back surface of FPC8,side walls 5 are shear-deformed, and by the pressure generated by theside wall deformation, ink in ink channel 4 is ejected from nozzle hole3 formed in nozzle plate 2.

As for metal electrodes 9, metals such as platinum, gold, silver,copper, aluminum, palladium, nickel, tantalum, and titanium can be used,and especially from the viewpoint of electrical characteristics andworkability, gold, aluminum, copper, and nickel are preferably used toform metal electrodes 9 employing a plating, evaporation or spatteringprocess. Above all, electroless plating is preferable. The piezoelectricelement is structured of particles having a diameter of several μm, andthe surface is formed rough, so that uniform electrodes cannot be formedon it by a slanted evaporation process using aluminum, and the drivevoltage in this case tends to be increased. On the other hand, byemploying electroless plating, since metal is precipitated uniformlyalong the base surface to form uniform electrodes, the drive voltage canbe lowered compared to the case of evaporation. For this electrolessplating nickel is preferably used.

The present invention is characterized in that cover plate 6, whichdirectly contacts actuator substrate 1, is made of machinable ceramicshaving higher thermal conductivity than piezoelectric elements 1 a and 1b, and having linear expansion coefficient Lc, which satisfies therelationship of |Lc−Lp|≦5×10⁻⁶/° C. where Lp represents the linearexpansion coefficient of piezoelectric elements 1 a and 1 b. Further onsaid cover plate 6, top plate 11, having higher thermal conductivitythan cover plate 6, is provided.

Regarding cover plate 6, by using a material of higher thermalconductivity than piezoelectric elements 1 a and 1 b, the heat generatedin said piezoelectric elements can be transferred to cover plate 6, andthe temperature raise of ink in ink channel 4 can be decreased. Forexample, in the case where PZT is used for piezoelectric elements 1 aand 1 b, since the thermal conductivity of PZT is 1.5-2.0 W/mK,machinable ceramics having higher thermal conductivity than this can beused. The thermal conductivity of cover plate 6 is preferably higherthan that of piezoelectric elements 1 a and 1 b by at least 5 W/mK, andmore preferably by at least 10 W/mK and most preferably by at least 50W/mK.

The machinable ceramics is generally called free-machining ceramics, andis the ceramics which is easily machined. Generally, ceramics haveextremely bad workability, and even when a costly diamond cutter isused, working efficiency remains extremely low and results in thedisadvantage of high manufacturing cost. However, the machinableceramics can be machined by machining apparatuses and tools for usualmetal machining.

Of the machinable ceramics, there are mica-glass ceramics and aluminumnitride, in which numerous minute cracks exist. The mica-glass ceramicshave a structure such that crystallites of fluorinated mica areprecipitated in three-dimension and in a disarrayed state in a glassymatrix. When the edge of a grinding tool cuts into this ceramics, themica crystallites are broken by priority, the breakages are held towithin a restricted area, and minute swarf particles are ejected. As forconcrete examples of the mica-glass ceramics, there are “Macor” made byCorning Glass Works Incorporated, and “Photoveel II” made by SUMIKINCERAMICS & QUARTZ Co., Ltd.

Aluminum nitride is a type of ceramics, which generates numerousinterior minute cracks, featuring excellent workability, and isespecially preferable due to its sufficiently high thermal conductivity.Specifically, AlN-BN (Aluminumnitride-Boronnitride) composed of at least70 mol % of Aluminum and less than 30 mol % of Boron is preferable.Specifically, there are AlN-BN made by SUMIKIN CERAMICS & QUARTZ Co.,Ltd, and “Sapal Msoft” made by TOKUYAMA Corp.

Further, a MgO—SiO₂ type mica ceramics, there is a “FORSTERITE” made byKyocera Corporation.

The machinable ceramics to be used for cover plate 6 necessarily havethe linear expansion coefficient Lc, which satisfies the relationship of|Lc−Lp|≦5×10⁻⁶/° C. where Lp represents the linear expansion coefficientof piezoelectric elements 1 a and 1 b. Namely it is preferable for thecover plate to have a near value of a linear expansion coefficient asthat of piezoelectric elements 1 a and 1 b. The reason for this is thatif the linear expansion coefficient of cover plate 6 is far differentfrom that of piezoelectric elements 1 a and 1 b, when cover plate 6 isadhered to actuator substrate 1 by using thermoset type adhesive agent,deformation can result during cooling leading to separation betweenmaterials. Further, by satisfying the above-mentioned relationship, theoccurrence of the similar deformation or separation as that mentionedabove, which can be caused by the heat generated in piezoelectricelements 1 a and 1 b can be prevented. More preferably, the relationshipof |Lc−Lp|≦3×10⁻⁶/° C. is satisfied.

The values of thermal conductivity w (W/mK) and linear expansioncoefficient Lc of the machinable ceramics shown as examples above are:

AlN-BN made by SUMIKIN CERAMICS & QUARTZ Co., Ltd : W=90, Lc=4.9 (roomtemperature to 500° C.),

“Sapal Msoft” made by TOKUYAMA Corp.: W=90, Lc=4.4,

“Photoveel II” made by SUMIKIN CERAMICS & QUARTZ Co., Ltd: W=19.5,Lc=1.4,

“FORSTERITE” made by Kyocera Corporation: W=7.8, Lc=10.5.

Each of these materials has a higher thermal conductivity than the PZT(W=1.5, Lp=5) preferably used for the piezoelectric elements, andsatisfies the above relationship of linear expansion coefficient.

On this cover plate 6, top plate 11, whose thermal conductivity ishigher than that of cover plate 6, is adhered via adhesive agent 12.This top plate 11 functions as the supporting body for attachingactuator substrate 1 adhered with cover plate 6 onto housing 13 providedin a carriage (not illustrated). Top plate 11 also functions as theguide member while wiping the surface of nozzle plate 2 by beingarranged to form approximately the same plane as the surface of nozzleplate 2. By constituting this top plate 11 with a member having higherthermal conductivity than that of cover plate 6, the heat generated inpiezoelectric elements 1 a and 1 b is easily transferred to cover plate6, having higher thermal conductivity than piezoelectric elements, andfurther, by making the heat transfer to top plate 11 having a higherthermal conductivity than cover plate 6, this top plate 11 functions asa so-called heat sink. For this purpose, top plate 11 is preferable tohave sufficiently larger surface area than cover plate 6, for ensuringsufficient heat capacity, and is preferable to have a wide contact areawith cover plate 6 for ensuring low thermal resistivity. For example,the thickness of top plate 11 in the longitudinal direction of inkchannel 4, which is parallel to the contact plane between cover plate 6and top plate 11, is preferably 1.0-10.0 mm.

Materials having higher thermal conductivity than cover plate 6 areusable for top plate 11, and aluminum (thermal conductivity W=236 W/mK),brass (W=106), copper (W=403), and diecast aluminum (W=90) arepreferably used. Above all, aluminum is preferable from the viewpoint ofworkability and cost.

In order to effectively transfer the heat generated in piezoelectricelements 1 a and 1 b, through cover plate 6 into top plate 11, adhesiveagent 12 for adhering cover plate 6 and top plate 11 is preferable tohave high thermal conductivity. From this viewpoint, regarding adhesiveagent 12 for adhering cover plate 6 and top plate 11, it is preferableto add Ag (silver) particles into the epoxy type adhesive agent.Although, epoxy type adhesive agent has extremely low thermalconductivity (0.1-0.3 W/mK), by adding Ag particles, the thermalconductivity of adhesive agent is improved to 3-5 W/mK. Generally, heattransfer resistance Rc of members having a thermal conductivity “k”(W/mK), thickness L (m), area A (m²) is represented by Rc=L/k·A (K/W),therefore, it is preferable to make L as small as possible while makingA as large as possible.

For this reason, the thinner the layer thickness of adhesive agent 12,the more preferable it is since the heat transfer resistance decreases.Further the surface of top plate 11 is preferable to be rough forimproving thermal dissipation, however, if air bubbles intervene theadhesive layer the thermal transfer resistance becomes very high becauseof the extremely low thermal conductivity of air, therefore, it ispreferable to make the layer thickness of adhesive agent 12 between topplate 11 and cover plate 6 to be 50-70 μm so as to prevent air bubblesfrom entering.

Further, the thermal conductivity of adhesive agent 14 existing betweencover plate 6 and actuator substrate 1 is preferable to have highthermal conductivity. Since this adhesive agent 14 contacts metalelectrodes 9, electrical insulation is required of adhesive agent 14,and the adhesive agent made by adding fine particles, having highthermal conductivity, such as AlN, alumina, or silica into epoxy typeadhesive agent. Although epoxy type adhesive agents have extremely lowthermal conductivity (0.1-0.3 W/mK), by adding ceramic particles of highthermal conductivity such as aluminum nitride (AlN), alumina, or silicaparticles, the thermal conductivity of adhesive agent 14 is improved (to0.5-1.0 W/mK in the case of AlN), and the heat in actuator substrate 1is effectively transferred to cover plate 6. The thickness of theadhesive agent between cover plate 6 and actuator substrate 1 ispreferable to be 5-10 μm. If it is thinner than 5 μm, air bubbles arelikely to enter into the adhesive layer, and if it is thicker than 10μm, the deformation efficiency of ink channel 4 decreases.

In the process of adhering cover plate 6 to actuator substrate 1,surface roughness of respective surfaces is important. In the case wherethe cover plate is formed of Photoveel II, since the adhesive agent isabsorbed into minute voids existing in the cover plate, the thickness ofthe adhesive agent layer after hardening can not maintained, and resultsin poor adhesive strength. In cases of Al₂O₃ and PZT, it is alsodifficult to obtain an optimum adhering condition, and manufacturingefficiency becomes rather poor.

On the other hand, since AlN-Bn has a dense material structure, smallamount of minute voids, large Young's modulus and high hardness, almostall the applied adhesive agent functions as the effective adhesive layerafter hardening. Therefore, AlN-BN is a material suitable for the coverplate exhibiting a high manufacturing efficiency.

In the present invention, the dielectric constant (ε) of cover plate 6is preferable to be low. Because the lower the dielectric constant, theless the electric field leakage from piezoelectric elements 1 a and 1 b,and the drive voltage can be lowered. If the drive voltage is lowered,the heat generated in piezoelectric elements 1 a and 1 b can bedecreased. Specifically, the dielectric constant (ε) of cover plate 6 isnot to be greater than 100, and preferably is not greater than 10. Forexample dielectric constant (ε) of AlN-BN made by SUMIKIN CERAMICS &QUARTS Co., Ltd is: ε=7.1, “Photoveel II” made by SUMIKIN CERAMICS &QUARTZ Co., Ltd: ε=approx. 6, and “FORSTERITE” made by KyoceraCorporation: ε=6.8, these values are extremely small compared to thedielectric constant of PZT (ε=2000-4000).

Further, the greater flexural strength of cover plate 6 is the morepreferable, specifically, the flexural strength of not less than 100 Mpais preferable, while not less than 200 Mpa is more preferable. Thereason for this is that when actuator substrate 1 is deformed, thesmaller the deflection of cover plate 6, the better the ink ejectingefficiency which can be attained. For example, the flexural strength ofAlN-BN made by SUMIKIN CERAMICS & QUARTZ Co., Ltd is 294 Mpa, theflexural strength of “Photoveel II” also made by SUMIKIN CERAMICS &QUARTZ Co., Ltd is 440 Mpa. If flexural strength is less than 100 MPa,the ink ejecting efficiency decreases and drive voltage must beincreased, which is not preferable.

Furthermore, Young's modulus of cover plate 6 is preferable to be 50-200GPa. Since the Young's modulus of piezoelectric elements 1 a and 1 b isabout 50 GPa, if the Young's modulus of cover plate 6 is greater than 50Gpa, cover plate 6 is not likely to be influenced by the deformation ofpiezoelectric elements 1 a and 1 b, and the ink ejecting efficiencyimproves. However, if Young's modulus of cover plate 6 is greater than200 Gpa, machinable characteristics of cover plate 6 lowered and cuttingproperty is degraded.

The Vickers hardness of cover plate 6 is preferable to be not greaterthan 5.0 GPa. If this exceeds 5.0 Gpa, the grinding property becomesworse, and cover plate 6 cannot be cut finely, and in addition the lifeof diamond cutters used for machining the plate is shortened.

Further, nozzle plate 2 is also preferable to have a higher thermalconductivity than piezoelectric elements la and lb. Since this nozzleplate is adhered onto the front edge surfaces of actuator substrate 1and cover plate 6, by making the thermal conductivity of nozzle plate 2higher than that of piezoelectric elements 1 a and 1 b, the heatgenerated in piezoelectric elements 1 a and 1 b can be dissipated fromnozzle plate 6 in addition to the dissipation from cover plate 6.

Regarding a material of this nozzle plate 2, the material having ahigher thermal conductivity than piezoelectric elements 1 a and 1 b canbe used. Since resins generally have lower thermal conductivity than thepiezoelectric elements, resins are not preferable, but metals arepreferable for the material of nozzle plate 2. For example, since thethermal conductivity of stainless steel is 15 W/mK, compared to thethermal conductivity of polyimide resin (0.1-0.2 W/mK), which iscommonly used for the nozzle plate material, the heat dissipation effectcan be improved by using stainless steel for nozzle plate 2. Above all,Covar or 42-alloy, which have values near the linear expansioncoefficient of piezoelectric substrate 1, is preferable for the nozzleplate material.

Embodiments EXAMPLE 1

On a polarized piezoelectric element “H 5 D” made by Sumitomo MetalIndustries, Ltd. (with a thermal conductivity W=1.5 W/mK, and a linearexpansion coefficient=5×10⁻⁶/° C.), 768 grooves, which are to be inkchannels and having a width of 80 μm, a depth of 200 μm, and a length of60 mm, are formed by using a diamond cutter to form the actuatorsubstrate. Further, nickel electrodes are formed by electroless platingon the side walls in these ink channels.

On the actuator substrate, a cover plate formed ofaluminumnitride-boronnitride AlN-BN (made by SUMIKIN CERAMICS & QUARTSCo., Ltd. having a thermal conductivity W=90.0 W/mK, and linearexpansion coefficient=4.9×10⁻⁶/° C.) is adhered by using a highthermo-conductive epoxy adhesive agent including AlN particles (at alayer thickness of 10 μm, W=1.0 W/mK).

Obtained adhered member is cut with a diamond cutter to form 2.5 mmlength ink channels, next, a nozzle plate made of polyimide resin and amanifold are adhered, after which a FPC (Flexible Printed Circuit) isconnected to each electrode. On the upper surface of the cover plate, analuminum top plate (1 mm thick) is adhered by using highthermo-conductive epoxy type resin including Ag particles (with a layerthickness of 50 μm, W=3.0 W/mK).

The recording head structure as described above is attached onto acarriage, and subjected to ejecting ink continuously for 2 seconds. Thevelocity of ink drops and the density of printed images at the time ofstarting the ejection and at the end of the ejection are measured.

EXAMPLE 2

As an adhesive agent, a normal epoxy type adhesive agent (W=0.3 W/mK)without added thermo-conductive particles was used. Other conditionswere the same as those of Example 1.

EXAMPLE 3

Electrodes are formed of aluminum, as an adhesive agent a normal epoxytype adhesive agent (W=0.3 W/mK) without thermo-conductive particles wasused, and a nozzle plate is formed of stainless steel. Other conditionswere the same as those of Example 1.

EXAMPLE 4

Electrodes were formed of aluminum, a cover plate formed of “PhotoveelII” (W=19.5 W/mK) made by SUMIKIN CERAMICS & QUARTZ Co., Ltd is adheredby using normal epoxy adhesive agent (W=0.3 W/mK) withoutthermo-conductive particles, and a nozzle plate is formed of stainlesssteel. Other conditions were the same as those of Example 1.

COMPARATIVE EXAMPLE 1

An alumina ceramics plate (W=33 W/mK) was used for the cover plate, andas an adhesive agent, normal epoxy adhesive agent (W=0.3 W/mK) withoutthermo-conductive particles was used. Other conditions were the same asthose of Example 1.

COMPARATIVE EXAMPLE 2

A depolarized piezoelectric material (PZT: thermal conductivity W=1.5W/mK) was used for the cover plate, an engineering plastic PEI(polyetherimide: W=0.1 W/mK) was used for the top plate, and as adhesiveagent normal epoxy adhesive agent (W=0.3 W/mK) without thermo-conductiveparticles was used. Other conditions were the same as those of Example1.

For an image quality evaluation, qualities of the printed images wereevaluated by visual observation using the following standard.

-   -   A: Without unevenness in the printed image, the image is very        crisp.    -   B: Unevenness in the printed image is not conspicuous, and the        image is acceptably crisp.    -   C: Unevenness in the printed image print is somewhat        conspicuous, and the image quality is somewhat degraded.    -   D: Unevenness in the printed image is conspicuous, and the image        quality is poor.

Further, a manufacturing efficiency (or easiness of production) wasevaluated for the recording head made in the above examples andcomparative examples, using the following standard.

-   -   A: Good manufacturing efficiency in the process of machining and        adhering, without requiring a polishing work for the ground cut        surface of the member constituted of adhered the piezoelectric        elements and a cover plate.    -   C: Somewhat degraded manufacturing efficiency in the process of        machining and adhering, requiring a polishing process.

Incidentally, adhesive 1 in Table 1 represents the adhesive agentbetween the top plate and the cover plate, and adhesive 2 represents theadhesive agent between the cover plate and the piezoelectric substrate.

In Example 1, when inkjet printing was conducted from edge to edge of a1350 mm wide large-sized recording medium, requiring 2 seconds for edgeto edge printing, the print density increase caused by heat generationin the head was 0.005, and unevenness in printing was not observed.Further, when ink channels of predetermined length were formed bycutting with a diamond cutter, the cut surface was clean and crisp didnot require a polishing process.

In Example 2, since an adhesive agent with low thermal conductivity wasused, the density difference became a little greater compared to Example1, however it was not detectable by visual observation and there was nopractical problem in image quality.

In Example 3, since aluminum was used for electrodes, the drive voltageof the recording head needed to be increased, and the density differencewas also increased to some extent, however, it was hardly detected byvisual observation, and there was no practical problem in image quality.

In Example 4, since the cover plate was different from that of Example1, the thermal conductivity was a little lower than in Example 1, andthe density difference increased, however, it was hardly detected byvisual observation, and there was no practical problem in image quality.

In Comparative Example 1, since Al₂O₃, which exhibits poor workabilitywas used for the cover plate, when ink channels of predetermined lengthwere formed by cutting with a diamond cutter, the cut surface was rough,which required a polishing process. Further, since a thermalconductivity of Al₂O₃ is less compared to AlN-BN, the density differencebecame very large and was detectable by visual observation, and theimage quality was poor.

In Comparative Example 2, since PZT of low thermal conductivity was usedfor the cover plate, and PEI with low thermal conductivity was used forthe top plate, the density difference was further increased.

Regarding the evaluation of manufacturing efficiency for the recordinghead, examples 1 to 3, where machinable ceramics are used for the coverplate, showed good efficiency, and other example or comparative exampleshowed somewhat degraded manufacturing efficiency.

Incidentally, adhesive 1 in Table 1 represents the adhesive agentbetween the top plate and the cover plate, and adhesive 2 represents theadhesive agent between the cover plate and the piezoelectric substrate.

EFFECT OF THE INVENTION

According to the present invention, an inkjet recording head withexcellent heat dissipation, with no noticeable density unevenness,exhibiting no deformation or separation, can be provided. And furthercan be provided an inkjet recording head exhibiting high reliability,which does not need in its manufacturing process a polishing work forthe ground cut surface of the member constituted of adhered thepiezoelectric elements and a cover plate.

1. An inkjet recording head for ejecting ink in ink channels bydeformation of the piezoelectric element, comprising: a partition walland a bottom wall, respectively formed by making grooves in an actuatorsubstrate made of a polarized piezoelectric element, for forminglongitudinal side surface and bottom surface of a plurality of inkchannels; a cover plate adhered on an upper surface of the partitionwall to form a top surface opposing to the bottom surface of theplurality of ink channels; a nozzle plate, adhered on a front sidesurface of the actuator substrate, having a nozzle hole for ejectingink, wherein the cover plate is made of a machinable ceramics, which hasa higher thermal conductivity than that of the piezoelectric element,and where Lc and Lp respectively represent the linear thermal expansioncoefficient of the cover plate and the piezoelectric element, therelationship of,|Lc−Lp|≦5 ×10⁻⁶/° C. is satisfied, and a top plate having higher thermalconductivity than the cover plate is adhered with an adhesive agent onthe cover plate, wherein a Vickers hardness of the cover plate is notgreater than 5.0 GPa.
 2. The inkjet recording head of claim 1, wherein aYoung's modulus of the cover plate is 50-200 GPa.
 3. The inkjetrecording head of claim 1, wherein a flexural strength of the coverplate is not less than 100 Mpa.
 4. The inkjet recording head of claim 1,wherein a dielectric constant (ε) of the cover plate is not greater than100.
 5. The inkjet recording head of claim 1, wherein a top plate is asupport member for mounting the recording head onto a carriage.
 6. Theinkjet recording head of claim 5, wherein a thickness of the top plateis 1.0 to 10.0 mm.
 7. The inkjet recording head of claim 1, wherein anadhesive agent used for adhering the cover plate and the top platecomprises epoxy type adhesive agent added with Ag particles.
 8. Theinkjet recording head of claim 1, wherein an adhesive agent used foradhering the cover plate and the upper surface of the partition wallcomprises an epoxy type adhesive agent added with one of aluminumnitride, alumina, or silica.
 9. The inkjet recording head of claim 1,wherein a thickness of the adhesive layer between the top plate and thecover plate is 50-70 μm, and a thickness of the adhesive layer betweenthe upper surface of the partition wall and the cover plate is 5-10 μm.